1. Field of the Invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus that has a gear axle.
2. Background Information
Heat transfer printers are known as an example of conventional image forming apparatuses. Such heat transfer printer is generally provided with a motor and a feed roller. A motor gear is mounted on the rotating axle of the motor. The heat transfer printer is also provided with a drive transmission gear and a feed roller intermediate gear between the motor gear and the feed roller gear. The drive force from the motor is transmitted to the feed roller gear and the feed roller via the drive transmission gear and the feed roller intermediate gear.
The feed roller intermediate gear is rotatably supported by an intermediate gear support axle mounted on the chassis. Also, the intermediate gear support axle has a mounting axle unit that is mounted on the chassis, and a bearing engagement part that rotatably supports the feed roller intermediate gear.
The feed roller intermediate gear meshes with the feed roller gear. A circular bearing unit 113a is provided to the feed roller intermediate gear 113, as shown in FIG. 25. A circular bearing engagement part 119b of the intermediate gear support axle 119 is inserted through the circular bearing unit 113a of the feed roller intermediate gear 113. The circular bearing unit 113a of the feed roller intermediate gear 113 contacts the circular bearing engagement part 119b of the intermediate gear support axle 119 at one location (Y) on the peripheral surface of the circular bearing engagement part 119b of the intermediate gear support axle 119, as shown in FIG. 25.
Also, the feed roller intermediate gear 113 has a small gear that meshes with the feed roller gear 106, and a large gear that meshes with the drive transmission gear 112, as shown in FIG. 25. The motor gear meshes with the large gear of the drive transmission gear 112, thereby transmitting the driving force from the motor gear to the feed roller gear 106 via the drive transmission gear 112 and the feed roller intermediate gear 113.
When the drive force is transmitted from the drive transmission gear 112 to the feed roller gear 106 via the feed roller intermediate gear 113, the feed roller intermediate gear 113 receives a force F8 from the drive transmission gear 112 that rotates in the R direction, and a reaction F9 in rotating the feed roller gear 106 in the T direction, which together create a resultant force F7, as shown in FIG. 25. The circular bearing unit 113a of the feed roller intermediate gear 113 is thereby pressed against the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119 along the line of force F7.
Accordingly, a frictional force μF7 (where μ is a dynamic friction coefficient) is applied between the bearing engagement part 119b and the bearing unit 113a of the feed roller intermediate gear 113. Due to this frictional force μF7, the position at which the bearing unit 113a comes into contact with the bearing engagement part 119b moves to the Y position in FIG. 25, which is inclined at an angle θ3 in the rotation direction S of the feed roller intermediate gear 113 along the peripheral surface of the bearing engagement part 119b. 
At the contact point Y, the frictional force μF7 and the resultant force balance each other out. In other words, the rotation angle θ3 at the contact point Y satisfies the following equation:μF7=F7 tan θ3
Thus, the point of contact is prevented from moving any further. The bearing unit 113a of the feed roller intermediate gear 113 is thereby rotated while maintaining contact with the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119 at the contact position Y.
When paper is conveyed in the opposite direction during the non-printing operation, the feed roller gear 106 rotates in the direction of the arrow X as the drive transmission gear 112 and the feed roller intermediate gear 113 rotate in the directions of the arrows V and W in FIG. 26, respectively.
At this time, the feed roller intermediate gear 113 receives a force F11 from the drive transmission gear 112, and a force F12 as a reaction in rotating the feed roller gear 106, which together create a resultant force F10, as shown in FIG. 26. The circular bearing unit 113a of the feed roller intermediate gear 113 is thereby pressed against the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119 along the line of force F10. Accordingly, a frictional force μF10 (where μ is a dynamic friction coefficient) is applied between the bearing engagement part 119b and the bearing unit 113a of the feed roller intermediate gear 113. As a result of this frictional force μF10, the position at which the bearing unit 113a comes into contact with the bearing engagement part 119b moves along the line of force F10 to the Y position in FIG. 26, which is inclined at an angle θ4 in the W rotation direction of the feed roller intermediate gear 113 along the peripheral surface of the bearing engagement part 119b. 
At the contact position Y, the frictional force μF10 that acts to move the point of contact and the force that prevents this movement balance each other out. In other words, the rotation angle θ4 at the contact point Y satisfies the following equation:μF10=F10 tan θ4
Thus, as a result of inclination at this angle θ4, the point of contact is prevented from moving any further. The bearing unit 113a of the feed roller intermediate gear 113 is thereby rotated while maintaining contact with the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119 at the contact position Y.
The conventional heat transfer printer shown in FIGS. 25 through 26 has drawbacks in that during printing, the magnitude of the frictional force μF7 that is applied between the bearing unit 113a of the feed roller intermediate gear 113 and the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119 changes as the dynamic frictional coefficient μ changes due to fluctuations in the force F7 created by the rotation of the feed roller intermediate gear 113. Accordingly, the position Y where the bearing unit 113a comes into contact with the bearing engagement part 119b shifts left and right along the inner peripheral surface of the bearing engagement part 119b. 
Specifically, this is disadvantageous in that the angle θ3 shown in FIG. 25 increases when the magnitude of the frictional force μF7 that is applied between the bearing unit 113a and the peripheral surface of the bearing engagement part 119b due to rotation increases as the force F7 increases since the dynamic friction coefficient μ changes as the force F7 changes. It is also disadvantageous in that the angle θ3 shown in FIG. 25 decreases when the magnitude of the frictional force μF7 that is applied between the bearing unit 113a and the peripheral surface of the bearing engagement part 119b due to the rotation decreases as the force F7 decreases. When the contact position Y moves, the feed roller intermediate gear 113 does not rotate, and moves instead along the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119, causing fluctuations in the amount of rotation of the feed roller intermediate gear 113.
Also, the heat transfer printer has drawbacks in that during the non-printing operation, the magnitude of the frictional force μF10 that is applied between the bearing unit 113a of the feed roller intermediate gear 113 and the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119 due to fluctuations in the force F10 that is applied changes due to the rotation of the feed roller intermediate gear 113 since the dynamic friction coefficient μ changes as the force F10 changes. (See FIG. 26). Accordingly, the position Y where the bearing unit 113a comes into contact with the bearing engagement part 119b moves left and right along the inner peripheral surface of the bearing engagement part 119b. 
Specifically, this is disadvantageous in that the angle θ4 in FIG. 26 increases when the magnitude of the frictional force μF10 that is applied between the bearing unit 113a and the peripheral surface of the bearing engagement part 119b increases as the force F10 increases. It is also disadvantageous in that the angle θ4 shown in FIG. 26 decreases when the magnitude of the frictional force μF10 that is applied between the bearing unit 113a and the peripheral surface of the bearing engagement part 119b due to the rotation decreases as the force F10 decreases. When the contact position Y moves, the feed roller intermediate gear 113 does not rotate, and instead moves along the peripheral surface of the bearing engagement part 119b of the intermediate gear support axle 119, causing fluctuations in the amount of rotation of the feed roller intermediate gear 113.
As described above, when the angle θ3 or θ4 by which the feed roller intermediate gear 113 rotates fluctuates, the distance by which the paper (not shown) is conveyed also fluctuates because of the fluctuations in the rotation amount of the feed roller gear 106. Also, the accumulated fluctuations also increase proportionately when the number of intermediate gears is increased, and the paper is therefore fed even more non-uniformly. This is disadvantageous in that it becomes difficult to precisely control the conveyance of paper with the feed roller 102.
In conventional practice, the fit tolerance between the bearing unit 113a of the feed roller intermediate gear 113 and the bearing engagement part 119b of the intermediate gear support axle 119 is minimized in order to prevent such non-uniformities the paper feeding described above. Therefore, component precision must be improved, and, as a result, component costs tend to increase.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved image forming apparatus that overcomes the problems of conventional art. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.